The present invention relates to methods of using the three-dimensional quadrupole ion trap mass spectrometer ("ion trap") which was initially described by Paul, et al.; see, U.S. Pat. No. 2,939,952. In recent years, use of the ion trap mass spectrometer has grown dramatically, in part due to its relatively low cost, ease of manufacture, and its unique ability to store ions over a large range of masses for relatively long periods of time. This latter feature makes the ion trap especially useful in isolating and manipulating individual ion species, as in a so-called tandem MS or "MS/MS" or MS.sup.n experiment where a "parent" ion species is isolated and fragmented or dissociated to create "daughter" ions, which may then be identified using traditional ion trap detection methods or further fragmented to create granddaughter ions, etc.
Isolation of individual ion species also has importance in other applications beside isolation of parent ions for MS/MS experiments. Given the relatively low cost and sensitivity of present-day commercial ion traps, they can be used to monitor for the presence of specific compounds or groups of related compounds, e.g., monitoring for the release of toxic gases in an production area. Controlling an ion trap to selectively isolate specific ion species of interest can be used to optimize the sensitivity of the trap for the selected species, which otherwise would be poorly detectable or completely undetectable.
As is well known, the quadrupole ion trap comprises a ring-shaped electrode and two end cap electrodes. Ideally, both the ring electrode and the end cap electrodes have hyperbolic surfaces that are coaxially aligned and symmetrically spaced. By placing a combination of AC and DC voltages (conventionally designated "V" and "U", respectively) on these electrodes, a quadrupole trapping field is created. A trapping field may be simply created by applying a fixed frequency (conventionally designated "f") AC voltage between the ring electrode and the end caps to create a quadrupole trapping field. The use of an additional DC voltage is optional, and in commercial embodiments of the ion trap a DC trapping voltage is not normally used. It is well known that by using an AC voltage of proper frequency and amplitude, a wide range of masses can be simultaneously trapped.
The mathematics of the quadrupole trapping field created by the ion trap were described in the original Paul, et al., patent. For a trap having a ring electrode of a given equatorial radius r.sub.0, with end cap electrodes displaced from the origin at the center of the trap along the axial line r=0 by a distance z.sub.0, and for given values of U, V and f, whether an ion of mass-to-charge ratio (m/e, also frequently designated m/z) will be trapped depends on the solution to the following two equations: ##EQU1## where .omega. is equal to 2.pi.f.
Solving these equations yields values of a.sub.z and q.sub.z for a given ion species having the selected m/e. If the point (a.sub.z, q.sub.z) maps inside the "stability envelop" for the ion trap, the ion will be trapped by the quadrupole field. If the point (a.sub.z, q.sub.z) falls outside the stability envelop, the ion will not be trapped and any such ions that are introduced within the ion trap will quickly move out of the trap. By changing the values of U, V or f one can affect the stability of a particular ion species. Note that from Eq. 1, when U=0, (i.e., when no DC voltage is applied to the trap), a.sub.z =0.
(It is common in the field to speak of the "mass" of an ion as shorthand for its mass-to-charge ratio. As a practical matter, most of the ions in an ion trap are singly ionized, such that the mass-to-charge ratio is the same as the mass. For convenience, this specification adopts the common practice, and generally uses the term "mass" as shorthand to mean mass-to-charge ratio.)
Each ion in the trapping field has a "secular" frequency which depends on the mass of the ion and on the trapping field parameters. As is well-known, it is possible to excite ions of a given mass that are stably held by the trapping field by applying a supplemental AC dipole voltage to the ion trap having a frequency equal to the secular frequency of the ion mass. Ions in the trap can be made to resonantly absorb energy in this manner. When the supplemental dipole voltage is relatively low, it can be used to cause ions of a specific mass to resonate within the trap, undergoing dissociating collisions within molecules of a background gas in the process. This technique, called collision induced dissociation or "CID," is commonly used in MS/MS to dissociate parent ions to create daughter ions. At higher voltages, sufficient energy is imparted by the supplemental voltage to cause those ions having a secular frequency matching the frequency of the supplemental voltage to leave the trap volume. This technique is now commonly used to eliminate unwanted ions from the ion trap, and to scan the trap to eject ions from the trap for detection by an external detector.
The typical basic method of using a commercial ion trap consists of applying an rf trapping voltage (V.sub.0) to the trap electrodes to establish a trapping field which will retain ions over a wide mass range, introducing a sample into the ion trap, ionizing the sample, and then scanning the contents of the trap so that the ions stored in the trap are ejected and detected in order of increasing mass. Typically, ions are ejected through perforations in one of the end cap electrodes and are detected with an electron multiplier. More elaborate experiments, such as MS/MS, generally build upon this basic technique, and often require the isolation of a specific ion mass in the ion trap.
Once the ions are formed and stored in the trap a number of techniques are available for isolating specific ions of interest. It is well-known that when the trapping field includes a DC component, the trapping field parameters (i.e., U, V and f) can be adjusted to isolate a single ion species, or a very narrow mass range, in the trap. A problem with this approach is that it is difficult to control the trapping field parameters with the high degree of precision, and it is difficult to calculate the precise combination of trapping field parameters needed to isolate a single mass or a narrow range of masses. Another problem is that most commercial ion traps do not have the ability to apply a DC trapping voltage, and adding this capability increases the amount and cost of the system hardware that is required. Finally, it is noted that when using this technique, the ions that are to be retained in the field will be near the edge of the stability boundary, so that the trapping efficiency is not optimal, and may be rather poor.
U.S. Pat. No. 4,736,101 describes another method of isolating an ion for MS/MS experiments. According to the technique taught by the '101 patent, a trapping field is established to trap ions having masses over a wide range. This is done in a conventional manner, as was well known in the art. Next, the trapping field is changed to eliminate ions other than the selected ion of interest. To do this the rf trapping voltage applied to the ion trap is ramped so as to cause ions of low mass to sequentially become unstable and be eliminated from the trap. The ramping of the rf trapping voltage is stopped at the point at which the mass just below the ion of interest is eliminated from the ion trap. The '101 patent does not teach how to manipulate the trapping field to eliminate ions having a mass that is higher than the mass of interest when no DC trapping voltage is applied. After the contents of the ion trap have been limited by the foregoing technique of changing the trapping voltage, the trapping voltage is relaxed so that, once again, ions over a broad range are trapped. Next, the parent ions within the ion trap are dissociated, preferably using CID, to form daughter ions. Finally, the ion trap is scanned by again ramping the quadrupole trapping voltage so that ions over the entire mass range sequentially become unstable and leave the trap.
The major deficiency of the method of the '101 patent is its failure to teach how to eliminate high mass ions from the trap without using a trapping field having a DC component. In addition, the technique of causing the low mass ions to be eliminated from the ion trap by instability scanning is also problematic. If m.sub.P is the mass to be retained in the trap, and the trapping field is manipulated to cause m.sub.P-1 to become unstable, then m.sub.P will, at that point, be very close to the stability boundary. Again, this may cause the trapping efficiency for m.sub.P to be quite low, and requires precise control of the trapping voltage as it is ramped to eliminate unwanted low mass ions.
Another method of isolating an individual ion species in an ion trap is described in U.S. Pat. No. 5,198,665 (the '665 patent) issued to the present inventor and coassigned herewith. (The disclosure of the '665 patent is hereby incorporated by reference.) According to the '665 patent, masses lower than the mass to be retained (m.sub.P) are first sequentially scanned out of the trap using resonance ejection. This has the advantage that m.sub.P-1 can be eliminated from the trap while m.sub.P is far from the stability boundary. After the low mass ions are so eliminated, a broadband supplemental signal is applied to the trap to eliminate the higher mass ions. The trapping voltage may be reduced slightly while applying the supplemental broadband voltage to bring ions just above m.sub.P into resonance. This technique is capable of producing highly accurate results. Since high mass ions remain in the trap while the low mass ions are being eliminated, a significant space charge remains. Unless proper measures are taken, this space charge can interfere with the accuracy of experiments using the technique.
It is also known in the prior art to apply various types of supplemental broadband voltage signals to the ion trap to simultaneously eliminate multiple unwanted ion species from the trap. The prior art generally teaches use of (1) broadband signals that are constructed from discrete frequency components corresponding to the resonant frequencies of the unwanted ions; and (2) broadband noise signals that essentially contain all frequencies, such that they act on the entire mass spectrum, and which are filtered to remove frequency components corresponding to the secular frequency(ies) of the ions that are to be retained in the ion trap. In all of the known prior art methods, the trapping field is held constant while the supplemental broadband voltage is applied to the ion trap. Examples of such techniques are shown in U.S. Pat. Nos. 5,134,286; 5,256,875; and 4,761,545.
None of the patents which teach the use of broadband excitation signals to eliminate unwanted ions from the ion trap en masse, adequately address the fact that the spacing of the secular frequencies of adjacent ion masses varies across the mass spectrum. For low masses, the secular frequencies of adjacent integer masses are far apart, whereas at high masses they are quite close. As a result, at low masses, if the ion of interest is not an integer mass, or if space charge or trapping field irregularities have caused a shift in the nominal secular frequency, there is a risk that the mass will not be excited and eliminated. On the other hand, in the high mass range, a single frequency component may cause resonance of multiple mass values, in which case a narrow "notch" in the broadband signal might not be sufficient to ensure that a desired ion will be retained in the ion trap.
A disadvantage of the prior art, which relies on waveforms containing a very large number of frequency components, is the high power requirements associated with having each of the frequency components present at sufficiently high power levels to cause excitation of ions across the mass spectrum. This disadvantage exists both for noise signals and for constructed waveforms, i.e., waveforms in which the frequency components are predetermined either by direct frequency selection or by an algorithm, such as an inverse Fourier transform of a frequency domain excitation spectrum to create a time domain excitation waveform. In a constructed waveform, it is important to further control the phases of the frequency components to minimize the dynamic range of the excitation waveform. As the number of frequency components increases, more elegant and time-consuming techniques are needed to create a time domain signal with a reasonable dynamic range, i.e., a minimized peak-to-peak voltage. For example, the '875 patent teaches a rather complex and time-consuming iterative technique for generating a supplemental voltage waveform.
Whatever technique is used to isolate a selected ion species in an ion trap, each of the methods uses essentially the same method for subsequently detecting the isolated species, i.e., scanning the contents of the trap. In the prior art method of scanning the contents of the trap, a supplemental AC voltage is applied across the end caps of the ion trap to create an oscillating dipole field supplemental to the quadrupole trapping field. (Sometimes the combination of the quadrupole trapping field and the supplemental rf dipole field is referred to as a "combined field.") In this scanning method, the supplemental AC voltage has a different frequency than the primary AC trapping voltage. The supplemental AC voltage causes trapped ions of specific mass to resonate at their secular frequency in the axial direction. When the secular frequency of an ion equals the frequency of the supplemental voltage, energy is efficiently absorbed by the ion. When enough energy is coupled into the ions of a specific mass in this manner, they are ejected from the trap in the axial direction where they are detected by a detector. The technique of using a supplemental dipole field to excite specific ion masses is sometimes called axial modulation.
In this prior art scanning method there are two ways of bringing ions of masses present in the trap into resonance with the supplemental AC voltage: scanning the frequency of the supplemental voltage in a fixed trapping field, or varying the magnitude V of the AC trapping voltage while holding the frequency of the supplemental voltage constant. Typically, when using axial modulation to scan the contents of an ion trap, the frequency of the supplemental AC voltage is held constant and V is ramped so that ions of successively higher mass are brought into resonance and ejected. The advantage of ramping the value of V is that it is relatively simple to perform and provides better linearity than can be attained by changing the frequency of the supplemental voltage. The method of scanning the trap by using a supplemental voltage will be referred to as resonance ejection scanning.
In commercial embodiments of the ion trap using resonance ejection as a scanning technique, the frequency of the supplemental AC voltage is set at approximately one half of the frequency of the AC trapping voltage. It can be shown that the relationship of the frequency of the trapping voltage and the supplemental voltage determines the value of q.sub.z (as defined in Eq. 2 above) of ions that are at resonance.
A technique commonly referred to as "mass instability scanning," described in U.S. Pat. No. 4,540,884, is also known in the prior art to scan the contents of the ion trap for detection and analysis. The '884 patent teaches scanning one or more of the basic trapping parameters of the quadrupole trapping field, i.e., U, V or f, to sequentially cause trapped ions to become unstable and leave the trap. The '884 patent teaches scanning a trapping parameter such that the unstable ions tend to leave in the axial direction where they can be detected using a number of techniques, for example, as mentioned above, a electron multiplier or Faraday collector connected to standard electronic amplifier circuitry. Nonetheless, resonance ejection scanning of trapped ions provides better sensitivity than can be attained using the mass instability technique taught by the '884 patent, and produces narrower, better defined peaks, i.e., resonance ejection scanning produces better overall mass resolution. Resonance ejection scanning also substantially increases the ability to analyze ions over a greater mass range.
Whichever method is used to scan the trap, ions are equally likely to move in either direction along the trap axis. Thus, half of the ions will move in the axial direction away from the detector and the other half will move toward the detector. This significantly limits the detection efficiency of the device. An additional disadvantage of the prior art resonance scanning technique can be seen by reference to FIG. 1. This figure shows the signal directly at the output of detector (i.e., before any filtering or other processing), resulting from a single scan of an isolated mass (perfluorotributylamine, "PFTBA," m/z=131). The divisions depicted on the horizontal axis are in increments of 50 .mu.sec, and the time required to scan the single isolated mass is approximately 180 .mu.sec. The high frequency oscillations that are apparent in the ion signal are the result of a frequency beating between the rf trapping voltage at 1050 kHz and the dipole supplemental ejection voltage at 485 kHz. The resulting beat frequency is 80 kHz. In the prior art, order to overcome the poor quality of the peak from a single scan, it has been necessary to average several scans in order to obtain a smooth peak with an accurately centered mass value. Such an averaged value, taken from many scans, is shown in FIG. 2. FIG. 3 shows the peak of FIG. 2 after it has been further processed by an integrator.
The flow from a GC is continuous, and a modem high resolution GC produces narrow peaks, sometimes lasting only a matter of seconds. In order to obtain a mass spectra of narrow peaks, it is necessary to perform at least one complete scan of the ion trap per second. The need to perform rapid scanning of the trap adds constraints which may also affect mass resolution and reproducibility. Similar constraints exist when using the ion trap with an LC or other continuously flowing, variable sample stream. Averaging scans in order to obtain accurate mass peaks reduces the scan cycle time and hence the number of different masses that can be monitored per unit time across a chromatographic peak. It is noted that the time for a single scan is more than just the scan time itself, since it must also include the ionization and ion isolation time, both of which are generally longer than the scan itself. Therefore, scan averaging for purposes of peak smoothing is an inherently inefficient process.